SCS-C02 Infrastructure Security Practice Question
A company wants to allow cross-account access to an S3 bucket. The bucket owner (Account A) wants to grant read-only access to users in Account B. Which combination of policies is required?
⚠ Common exam trap
SCS-C02 often tests the misconception that a bucket policy alone is sufficient for cross-account access, ignoring the requirement for an identity-based policy in the caller's account.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A bucket policy in Account A granting s3:GetObject to Account B and an IAM policy in Account B allowing s3:GetObject
Cross-account S3 access requires two-sided permission: the bucket owner (Account A) must grant access via a bucket policy that allows Account B's principal to perform s3:GetObject, and the IAM user or role in Account B must also have an IAM policy allowing s3:GetObject. Both are evaluated, and the request is allowed only if both sides permit it.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A bucket ACL in Account A granting READ access to Account B
Why it's wrong here
Bucket ACLs are a legacy authorization mechanism that predates IAM and do not integrate with IAM policies. Even if Account B is granted READ on the bucket ACL, READ corresponds to s3:ListBucket, not s3:GetObject, and ACLs cannot specify individual IAM principals or roles in another account. Cross-account object access requires a bucket policy that explicitly states the granted action and principal, so an ACL alone will not allow the GetObject call.
- ✓
A bucket policy in Account A granting s3:GetObject to Account B and an IAM policy in Account B allowing s3:GetObject
Why this is correct
Cross-account access is an AND operation: the bucket policy in Account A must explicitly allow the principal (the root user or a specific IAM principal in Account B) to call s3:GetObject, and the IAM identity in Account B must have an attached identity-based policy that also permits s3:GetObject. If either policy denies or lacks the permission, the request fails, because both the resource-based policy (in the owning account) and the identity-based policy (in the accessing account) must grant the action. This pattern is the standard way to grant direct cross-account access to an S3 object while keeping the resource owner in control.
- ✗
An IAM policy in Account A that allows s3:GetObject
Why it's wrong here
An IAM policy in Account A is evaluated only for principals in Account A; it has no effect on requests coming from users or roles in Account B. To permit a cross-account access, the bucket owner must add a resource-based policy on the S3 bucket that names the external account or principal as the principal. Without a bucket policy, the IAM policy in Account A cannot be attached to the Account B user, so the request will be denied regardless of what the policy allows.
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An IAM role in Account B that grants s3:GetObject to Account A
Why it's wrong here
Putting an IAM role in Account B is the wrong direction: roles used for cross-account access are placed in the resource-owning account (Account A) and trusted by the accessing account. Additionally, an IAM role's permission policy applies to the session of the principal that assumes the role, not to the entire Account A. Here the role would have no way to grant Account A itself the s3:GetObject permission on the bucket, because permissions are attached to roles and then assumed, not granted to another AWS account as a unit.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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